Step crystal growth on β-nucleated polypropylene homocomposites governing the crystalline compositions

Polymorphisms of polypropylene random copolymers (PPR) are usually regulated by the amplified β-nucleating effect of isotactic polypropylene (iPP). Nevertheless, the synergetic effect of β-nucleating agents (β-NAs) and iPP on growth of β-PPR crystals are still unclear to explain the debated content of β crystals obtained in the literatures. In this work, the growth mechanisms of PPR on β-nucleated iPP and the benchmarks were systematically revealed and discussed. First, a crystalline order is found existed in the PPR/iPP/β-NA ternary composite system. Then, the step crystal growth of each component is confirmed by the stepwise isothermal crystallization via microscopes. The results show that β-PPR crystallizes on the front of pre-crystallized β-iPP and is terminated by the adjacent α-PPR crystals. The overgrowth and spatial contact restrictions lay down a nucleation-controlled procedure. The morphologies and polymorph compositions can be regulated on the relative sizes of core and shell crystals, which are then determined by the size of β-iPP pre-crystallites, β-NA concentrations and the final heating temperature (Tf). At last, a high Kβ up to 0.5 can be easily reproduced at a low filling concentration of 0.3% β-NAs based on the nucleation-controlled β-growth. The easy preparation strategy of β-PPR that allows diversified thermal processing of PPR/iPP/β-NA composites enriches the researches on structure–performance relationship via macromolecular engineering.